In recent years, as a key device for high speed parallel optical information processing surface light emitting type semiconductor laser devices, which emit laser light perpendicular to the substrate, have been advanced.
FIG. 5 shows, in cross-section, a surface light emitting type semiconductor laser device having an AlAs/AlGaAs multilayer reflection mirror, disclosed in "Monthly Report of Science", vol.41, No.11, pp.910 to 913, by Professor Iga of the Tokyo Institute of Technology. In FIG. 5, 1 is an n type GaAs substrate. An n type Al.sub.0.3 Ga.sub.0.7 As etching stopping layer 2 is disposed on the substrate 1. An n type multilayer film 3 is disposed on the etching stopping layer 2. This multilayer film 3 comprises twenty pairs of AlAs and Al.sub.0.1 Ga.sub.0.9 As layers, where the thickness of the respective layers are: ##EQU1## For example, when the wavelength is 880 nm, the AlAs layer is 741 angstroms thickness and the Al.sub.0.1 Ga.sub.0.9 As layer is 625 angstroms thickness. A p type GaAs active layer 4 is disposed on the multilayer film 3. An active region 31 is disposed at the center of active layer 4. A p type Al.sub.0.3 Ga.sub.0.7 As cladding layer 5 is disposed on the active layer 4. A p type GaAs contact layer 6 is disposed on a portion of the cladding layer 5. A SiN insulating film 7 is disposed on the cladding layer 5 and a circular SiN insulating film 7b is disposed on a portion of the contact layer 6. A p side electrode 9a is disposed on the insulating films 7 and 7b, and an n side electrode 10a is disposed on the rear surface of substrate 1. Reference numeral 8 denotes a circular mesa groove. Reference numerals 26a and 26b denote crystal surfaces constituting a pair of resonator facets.
This surface light emitting type laser device will operate as follows.
Holes and electrons which are injected into the device from p side electrode 9a and n side electrode 10a are effectively confined in the active layer 4 by the heterojunction barrier between the GaAs active layer 4 and the Al.sub.0.3 Ga.sub.0.7 As cladding layer 5 and the heterojunction barrier between the GaAs active layer 4 and the multilayer film 3, and recombine to generate light having a wavelength corresponding to the energy band gap of the active layer 4. The generated light increases as the current level is increased. When the current reaches a certain value (threshold value), the gain exceeds the loss and produces laser oscillation, and light is emitted from the cavity facet 26a. In order to lower the threshold value, it is necessary to reduce the loss. In order to reduce the loss, a method of increasing the reflectances of the resonator facets 26a and 26b is considered. In this prior art laser device, the AlGaAs/AlAs multilayer film 3 and the circular SiN film 7b are provided to increase the reflectances of the resonator facets. Herein, the thickness of SiN film 7b is set to 1000 to 1900 angstroms.
In the prior art surface light emitting type laser device of such a construction, reduction of threshold value is attempted by utilizing the multilayer film 3 and the SiN film 7b. However, since there is no confinement structure for confining the carriers to the active region 31 which the carriers injected from the substrate side electrode 10a (in this device, electrons), are dispersed as shown in FIG. 7. A reactive current which does not contribute to the oscillation is generated, whereby the threshold current is likely to increase. In addition, since the reflectance of the multilayer film 3 is approximately the same for the different light of oscillation modes, the mode control is difficult.